I played about 150 rounds at the 2 Torrey Pines courses between 2005 and 2009, with probably 60% of those rounds on the North. In my very humble opinion, the North Course would benefit from a renovation, in the sense that it could use a "freshening up." Given freedom to change certain aspects of the course (mainly bunkers, as others have already suggested), I believe a professional architect could improve the course, but mostly in pretty subtle ways. Done right, I think the general public would appreciate such changes, assuming the work does not fundamentally alter the character of the course. However, regardless of the scope and quality of the work, if the city raises rates, the vast majority of locals will be extremely unhappy.
Regarding the course architecture, the oceanfront setting and the fact it is a great course for walkers are the best things TP North has going for it. The stunning views from many points on the course (such as #1 green, #2 tee, approach into #5, #6, #7) frame the course so beautifully, and Bell seemed to make better use of the cliffs/canyons on the North, which I think enhances the look and feel of certain holes, particularly for the everyday (non-gca enthusiast) golfer. That is not to say there are not strong golf holes on the merit of their design. My favorite holes are #4, #5, #7, #11, #13. The one place that I think is really in need of substantial re-design (and may even have a little space to use) is #17. I find it boring, ugly, weak, and out of character with the rest of the course. I also love that the North is a pretty easy (and beautiful) walk. There are a couple of hills that get your heart pumping a little, but the terrain is mostly manageable. Plus, the mental stress and frustration often encountered over on the South is nearly nonexistent on the North. I think that is another big reason locals love the North so much (great views, lower price, far easier scoring...adds up for most weekend warriors).
On the other hand, I don't think most tour players will change their opinion much, regardless of the changes made. Perhaps they'd appreciate a few flatter greens and wider fairways (and I can't think of why you would mess with the greens there), but for tour players that course is already too easy. As Pat Perez mentions, there is not much room to lengthen it, and I don't think lengthening the course would really accomplish anything anyway.
Mickelson may not be the right guy for the job, but he is a beloved local son, so it shouldn't shock anyone if he gets the nod. He has been lobbying to work on TP North for a few years now and from what I hear Gary Stephenson does some nice work. So, assuming Stephenson is significantly involved on the renovation project, I would be supportive. Of course, the city was a bit of a mess my whole 5 years in the area, so I would not be surprised if this were handled poorly.